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Title: Thermal induced nano-structural and optical changes of nc-Si:H deposited by hot-wire CVD


Article Type: Original Research - Nano Express


Keywords: Hot-wire CVD; quantum size effects; nano-crystallite; optical band gap


Corresponding Author: Dr Christopher Joseph Arendse, PhD


Corresponding Author's Institution: CSIR National Centre for Nano-Structured Materials, P. O. Box
395, Pretoria 0001, South Africa


First Author: Christopher Joseph Arendse, PhD


Order of Authors: Christopher Joseph Arendse, PhD; Gerald F Malgas, PhD; Theo F Muller, MSc;
Dirk Knoesen, PhD; Clive J Oliphant, MSc; Clive J Oliphant, MSc; David E Motaung, MSc; David E
Motaung, MSc; Gerald F Malgas, PhD; Bonex W Mwakikunga, MSc; Bonex W Mwakikunga, MSc;
Bonex W Mwakikunga, MSc
Manuscript
Click here to download Manuscript: NRL Manuscript_Arendse et al.doc                   Click here to view linked References

   1
   2
   3
   4                Thermal induced nano-structural and optical changes of nc-Si:H
   5
   6                deposited by hot-wire CVD
   7
   8
   9
  10
  11                C. J. Arendse1,, G. F. Malgas1, T. F. G. Muller2, D. Knoesen2, C. J.
  12
  13                Olpihant1,2, D. E. Motaung1,2 and B. W. Mwakikunga1,3,4
  14
  15
  16
  17                1
  18                    CSIR National Centre for Nano-Structured Materials, P. O. Box 395, Pretoria
  19
  20                    0001, South Africa
  21
  22                2
                        Department of Physics, University of the Western Cape, Private Bag X17,
  23
  24
  25                    Bellville 7535, South Africa
  26
                    3
  27                    School of Physics, University of the Witwatersrand, Private Bag 3, P. O.
  28
  29                    Wits, Johannesburg 2050, South Africa
  30
  31                4
  32                    Department of Physics, University of Malawi, The Polytechnic, Private Bag
  33
  34                    303, Blantyre, Malawi
  35
  36
  37
  38
                    Abstract
  39
  40
  41                We report on the thermal induced changes of the nano-structural and optical
  42
  43                properties of hydrogenated nanocrystalline silicon in the temperature range
  44
  45                200  700 C. The as-deposited sample has a high crystalline volume fraction
  46
  47
  48                of 53% with an average crystallite size of  3.9 nm, where 66% of the total
  49
  50                hydrogen is bonded as SiH monohydrides on the nano-crystallite surface. A
  51
  52
  53                growth in the native crystallite size and crystalline volume fraction occurs at
  54
  55                annealing temperatures  400 C, where hydrogen is initially removed from
  56
  57                the crystallite grain boundaries followed by its removal from the amorphous
  58
  59                
  60                 Corresponding author: C. J. Arendse (CArendse@csir.co.za)
  61                Tel: +27 12 841 3671, Fax: +27 12 841 2229
  62
  63
  64                                                                                              1
  65
 1
 2
 3
 4   network. The nucleation of smaller nano-crystallites at higher temperatures
 5
 6   accounts for the enhanced porous structure and the increase in the optical
 7
 8
 9
     band gap and average gap.
10
11
12
13   Keywords
14
15   Hot-wire CVD, quantum size effects, nano-crystallite, optical band gap
16
17
18
19
20   1. Introduction
21
22
23
24
25    Hydrogenated nanocrystalline silicon (nc-Si:H) has been the subject of
26
27   intense scientific and technological interest over the past decade, mainly due
28
29   to   its   reduced    photo-induced      degradation     [1],     efficient   visible
30
31
     photoluminescence [2], tailored optical band gap [3], increased conductivity
32
33
34   and greater doping efficiency [4]. It has been highlighted that these unique
35
36   features are a direct cause of the quantum size effects of the silicon nano-
37
38   crystallites. These improvements make nc-Si:H a potential candidate for
39
40
41   application in photovoltaic and opto-electronic devices [5, 6].
42
43
44
45    The hot-wire chemical vapour deposition (HWCVD) technique, based on the
46
47
     catalytic decomposition of the precursor gasses by a heated transition metal
48
49
50   filament, has been established as a viable deposition technique for nc-Si:H
51
52   thin films [6-7]. The structural and opto-electronic properties of the thin films
53
54   are dependent on the deposition parameters, of which the hydrogen dilution
55
56
57   and substrate temperature are the most crucial. It has been established that
58
59   the etching effect of atomic hydrogen, created by the catalytic decomposition
60
61
62
63
64                                                                                      2
65
 1
 2
 3
 4   of H2, is responsible for the termination of weak SiSi bonds from the surface
 5
 6   and sub-surface regions and that the nucleation of the nano-crystallites are
 7
 8
 9   improved by increasing the hydrogen dilution [7-10]. It has also been reported
10
11   that the hydrogen dilution during deposition determines the concentration and
12
13   the distribution of hydrogen in nc-Si:H, which is closely related to the nano-
14
15
16   structural features; i.e. crystallite size and crystalline volume fraction [11-14].
17
18   These nano-structural features eventually determine the optical properties of
19
20   the material. In particular, the quantum size effects of the Si nano-crystallites
21
22   and the hydrogen concentration have a strong correlation with the optical
23
24
25   band gap [15-16].
26
27
28
29    An investigation into the role of hydrogen in nc-Si:H is therefore crucial for
30
31
32   the understanding of its relation to the nano-structure and the optical
33
34   properties. In this contribution, we investigate the effects of the hydrogen
35
36   concentration and bonding configuration in nc-Si:H deposited by HWCVD on
37
38
     the nano-structural features and the optical properties. The hydrogen
39
40
41   concentration and bonding configuration were controlled by post-deposition
42
43   isochronal annealing.
44
45
46
47
48   2. Experimental
49
50
51
52    The nc-Si:H thin film was deposited by the HWCVD process simultaneously
53
54
     on single-side polished <100> crystalline silicon and Corning 7059 substrates,
55
56
57   using a mixture of 4 sccm SiH4 and 26 sccm H2 decomposed by seven
58
59   parallel tungsten filaments, 15 cm apart and 36 cm away from the substrates.
60
61
62
63
64                                                                                    3
65
 1
 2
 3
 4   A detailed description of the experimental set-up is given elsewhere [17-18].
 5
 6   The filament temperature, substrate temperature and deposition pressure
 7
 8
 9   were fixed at 1600 C, 420 C and 60 bar, respectively. The as-deposited
10
11   nc-Si:H thin film was  1140 nm-thick, as measured using a Veeco®
12
13   profilometer.
14
15
16
17
18    Subsequent annealing was performed under high-purity, flowing N2 gas in a
19
20   tube furnace at annealing temperatures (TA) ranging from 200 – 700 C in 100
21
22
23   C increments. The N2 flow rate, heating rate and dwell time for all
24
25   temperatures amounted to 300 sccm, 10C/min and 30 minutes, respectively.
26
27
28   After each annealing temperature, the thin film was allowed to cool to room
29
30   temperature in the tube furnace, while maintaining the N2 flow rate. Thereafter
31
32   the required analytical techniques were performed.
33
34
35
36
37    Fourier transform infrared (FTIR) absorption spectra were collected in
38
39   transmission geometry from 400 – 4000 cm-1 with a spectral resolution of 1
40
41   cm-1, using a Perkin-Elmer Spectrum 100 FTIR spectrophotometer. The
42
43
44   structural properties were investigated using a Jobin-Yvon HR800 micro-
45
46   Raman spectrometer in backscattering geometry at room temperature. The
47
48   Raman spectra were collected in the region 100 – 1000 cm-1 with a spectral
49
50
51
     resolution of 0.4 cm-1, using an excitation wavelength of 514.5 nm. X-ray
52
53   diffraction (XRD) spectra were collected in reflection geometry at 2-values
54
55   ranging from 10 – 90 with a step size of 0.02, using a Phillips PW 1830 x-ray
56
57
58   powder diffractometer operating at 45 kV and 40 mA. Copper K1 radiation
59
60   with a wavelength of 1.5406 Å was used as the x-ray source. Optical
61
62
63
64                                                                                4
65
 1
 2
 3
 4   transmission spectra were measured from 200 – 900 nm with a spectral
 5
 6   resolution of 1nm, using a Perkin-Elmer LS75S UV/VIS spectrophotometer.
 7
 8
 9
10
11   3. Results and Discussion
12
13
14
15   3.1 Nano-structural properties
16
17
18     FTIR spectroscopy is the established analytical technique of choice to
19
20   probe the silicon-hydrogen bonding configurations and to calculate the
21
22   hydrogen concentration in nc-Si:H and related material. The FTIR absorption
23
24
25   spectrum of the sample in the as-deposited state is shown in Fig. 1. The
26
27   strong absorption bands in the region 920 – 1250 cm-1 is associated with the
28
29   asymmetric SiOSi stretching vibration [19], whereas the peak centred
30
31
32   around 2250 cm-1 is assigned to the HSiO3 vibration [20]. This is indicative
33
34   of an oxidation effect caused by its porous-like microstructure, which is a
35
36   typical feature for nc-Si:H thin films [21-22]. The enhanced absorption band
37
38
39   centred around 640 cm-1 is attributed to the rocking vibrations of all bonding
40
41   configurations of SiHx [23]. The absorption bands in the region 1900 – 2150
42
43   cm-1 is a result of the convolution of several absorption bands associated with
44
45
46   the stretching vibrations of SiHx in different configurations. This is illustrated
47
48   in the insert of Fig. 1, where the absorption spectrum is decomposed into
49
50   three Gaussian components. The absorption peaks centred around 1985 cm-1
51
52
53   and 2090 cm-1 are assigned to the stretching vibrations of SiH
54
55   monohydrides in the amorphous network (isolated) and on the surface of the
56
57   Si nano-crystallites (clustered), respectively [21, 24]. The weak absorption
58
59
60
61
62
63
64                                                                                    5
65
 1
 2
 3
 4   band centred at  2130 cm-1 is assigned to the existence of (SiH2)n
 5
 6   polyhydride complexes on Si nano-crystallite grain boundaries [25-26].
 7
 8
 9
10
11      To quantify the fraction of H bonded on the surface of nano-crystallites in
12
13   nc-Si:H, we define a structure factor, R s  I 2090 / I1985  I 2090  I 2130  , where I
14
15
16   denotes that integrated intensity of each decomposed peak. The total bonded
17
18   hydrogen concentration (CH) was estimated from the integrated absorption of
19
20
     the 640 cm-1 rocking mode using previous reported procedures [27-28]. In the
21
22
23   as-deposited state, CH amounts to  2 at.%, characteristic for nc-Si:H
24
25   deposited with high hydrogen dilution [16, 29], where  66% thereof is bonded
26
27
28   on the surface of the nano-crystallites. We propose that this relatively high
29
30   value for Rs is indicative of a high crystalline volume fraction.
31
32
33
34
35
      Fig. 2 shows the plots of the hydrogen concentration and the structure factor
36
37   as a function of annealing temperature. The hydrogen concentration and
38
39   structure factor are relatively constant at temperatures below 400 C,
40
41
42
     demonstrating that the nano-structure is stable in this temperature regime.
43
44   After annealing at 400 C most of the (SiH2)n polyhydride bonds on the
45
46   grain boundaries of the Si nano-crystallites have been terminated and
47
48
49   consequently results in an increase in the structure factor. Annealing at higher
50
51   temperatures induce a significant decrease in CH, coupled with an increase in
52
53   Rs. These changes are caused by the preferential termination of the isolated
54
55
56
     SiH bonds in the amorphous network. We propose that the instability
57
58   induced at TA  400 C is related to the growth of the native nano-crystallites
59
60   and to the nucleation of nano-crystallites in the amorphous network, thereby
61
62
63
64                                                                                           6
65
 1
 2
 3
 4   resulting in an increase in the crystalline volume fraction. It should be noted
 5
 6   that no SiHx absorption peaks were identified after annealing at 700 C.
 7
 8
 9
10
11    Raman      spectroscopy       provides     direct   nano-structural      information
12
13   quantitatively related to the average nano-crystallite size and the crystalline
14
15
16   volume fraction in nc-Si:H. Fig. 3 shows the Raman spectra of the sample in
17
18   the as-deposited state and after annealing at specific temperatures. All
19
20   spectra display the following main features: (i) a sharp peak centred around
21
22   515 cm-1, associated with the transverse optic (TO) mode of the nc-Si phase;
23
24
25   (ii) the broad shoulder centred around 480 cm-1, due to the TO-mode of the
26
27   amorphous silicon (a-Si) phase; and (iii) a smaller shoulder around 505 cm-1,
28
29   corresponding to the distribution of crystalline grain boundaries in the sample.
30
31
32
33
34    The crystalline volume fraction, f c  [ A505  A515 ]/[ A480  A505  A515 ] , can be
35
36
37
     estimated from the integrated areas of the afore-mentioned deconvoluted
38
39   Gaussian peaks [30]. The crystallite size is empirically calculated from
40
41   d Raman  2π B/Δ , where  is the shift of the 515 cm-1 peak relative to the
42
43
44   c-Si peak at 520 cm-1 and B  2.0 cm-2 [31]. The quantitative Raman results
45
46   are summarized in Table 1. In the as-deposited state, the average crystallite
47
48
49   size and the crystalline volume fraction amounts to about 3.9 nm and 53%,
50
51   respectively, and remain relatively constant after annealing at 300 C. These
52
53   observations reiterate that the nano-structure of the sample remains stable at
54
55
56   temperatures below 400 C. A blue shift of the nc-Si TO-peak, accompanied
57
58   with a reduction in the intensity of the a-Si TO-peak is observed at annealing
59
60
61
     temperatures  400 C, indicative of an increase in the crystallite size and the
62
63
64                                                                                        7
65
 1
 2
 3
 4   crystalline volume fraction, respectively, and supports the claims based on the
 5
 6   FTIR results.
 7
 8
 9
10
11    XRD was employed as a complimentary method to qualitatively probe the
12
13   changes in the crystallinity as a function of annealing temperature (see Fig.
14
15   4). Three preferential orientations in the <111>, <200> and <311> directions
16
17
18   are observed. The crystallite size in the as-deposited state, estimated from
19
20   the full-width-half-maximum (FWHM) of the (111)-peak, amounts to  19.5 nm
21
22   [32]. A narrowing in the FWHM of the (111)-peak, accompanied with an
23
24
25   increase in its intensity is observed with an increase in annealing temperature.
26
27   This confirms the increase of the crystallite size and crystalline volume
28
29   fraction, as probed by Raman spectroscopy.
30
31
32
33
34    The thermal induced nano-structural changes of the nc-Si:H thin film can be
35
36   interpreted as follows, based on the variation of the SiHx bonding and the
37
38
39   crystalline character. In the as-deposited state the crystalline volume fraction
40
41   is relatively large and therefore the majority of H is bonded to the surface of
42
43   the   nano-crystallites.   The   nano-structural    properties   are   stable    at
44
45
46   temperatures below 400 C, attributed to its large crystalline volume fraction.
47
48   An initial increase in the native crystallite size is observed after annealing at
49
50   400 C, resulting in the removal of hydrogen from the grain boundaries. It is
51
52
53   also feasible that smaller crystallites have coalesced into larger crystallites. At
54
55   higher temperatures, hydrogen is removed preferentially from the amorphous
56
57   phase, indicative of the nucleation of smaller nano-crystallites of size  3 nm
58
59
60   in the amorphous network [33], undetected by Raman spectroscopy and XRD.
61
62
63
64                                                                                    8
65
 1
 2
 3
 4   3.2 Optical properties
 5
 6    The optical properties were determined from UV-visible transmission
 7
 8
 9
     measurements, using the method proposed by Swanepoel [34-35]. The
10
11   thickness of the as-deposited sample was calculated to be  1180 nm, which
12
13   concurs to that measured by profilometry. The refractive index n() of a
14
15
16   material is an important optical parameter, since it is directly proportional to
17
18   density [36]. Fig. 5 shows the spectral dependence of the calculated refractive
19
20   index for the sample in the as-deposited state and after annealing at specific
21
22
23   temperatures. A slight increase in the refractive index is observed after
24
25   annealing at 400 C, followed by a decrease at higher temperatures. The
26
27   initial increase can be ascribed to the increase in the native crystallite size
28
29
30   and possibly due to the coalescence of smaller nano-crystallites. Furthermore,
31
32   the presence of (SiH2)n complexes in the as-deposited state is indicative of
33
34   a disordered, porous material and the removal thereof after 400 C would
35
36
37   therefore result in a more compact material. The subsequent decrease of the
38
39   refractive index at higher temperatures is attributed to a more porous
40
41   structure, possibly caused by the nucleation of smaller nano-crystallites in the
42
43
44   amorphous network. Similar trends in the refractive index at zero photon
45
46   energy (no) are observed (see Table 1).
47
48
49
50
51
      Detailed analysis of the refractive index spectra were performed using the
52
53   suggested model of Wemple et al [37]. At energies below than of the optical
54
55   band gap, the refractive index is related to the square of the photon energy
56
57
     (h)2 by:
58
59
60
61
62
63
64                                                                                 9
65
 1
 2
 3
 4                                                 E ME D
 5                                n 2 ( ν)  1                                 (1)
 6
                                                  E  ( hν ) 2
                                                   2
                                                   M

 7
 8
 9
10   where EM and ED is the average gap and dispersion energy, respectively. The
11
12
13   plot of 1/[n2()-1] versus (h)2 allows for the determination of EM, ED and no.
14
15   The extrapolated results of no and EM, calculated from the linear fit through
16
17   the data, are listed in Table 1.
18
19
20
21
22    The spectral dependence of the absorption coefficient () for the sample in
23
24   the as-deposited state and after annealing at specific temperatures is
25
26
27   depicted in Fig. 6. The optical band gap, referred to as E04, is defined as the
28
29   photon energy where ()  104 cm-1, and the values thereof are reported in
30
31
     Table 1. A red shift in E04 is observed for TA  600 C followed by an
32
33
34   unexpected blue shift after annealing at 700 C. It is established that the
35
36   optical band gap of hydrogenated amorphous silicon (a-Si:H) deposited by
37
38
39
     HWCVD and PECVD increases with an increase in the hydrogen
40
41   concentration [7, 38]. It should be noted that the optical band gap for the
42
43   sample in the as-deposited state is larger than that of a-Si:H with similar CH
44
45   values. This discrepancy is due to the presence of nano-crystallites in the
46
47
48   amorphous network, which lowers the absorption in nc-Si:H and shifts the
49
50   optical band gap towards higher energies [15-16]. The quantum size effect
51
52   size also predicts that an increase in crystallite size is associated with a
53
54
55   decrease in the optical band gap. The initial decrease in E04 after 400 C is
56
57   due to the combined effect of the decreased CH and the growth in the
58
59   crystallite size. After annealing at 600 C the initial hydrogen concentration
60
61
62
63
64                                                                               10
65
 1
 2
 3
 4   has decreased by  90% with about the same incremental increase in the
 5
 6
     crystallite size as at 400 C, and therefore a more notable decrease in E04 is
 7
 8
 9   expected. However, a minute 0.03 eV decrease in E04 is observed and is
10
11   attributed to the nucleation of smaller nano-crystallites in the amorphous
12
13   network, which explains the competing increasing effect on E04. After
14
15
16   annealing at 700 C, where no hydrogen was detected by FTIR spectroscopy,
17
18   this effect is more pronounced in that an increase in the optical band gap is
19
20   observed.
21
22
23
24
25    The optical band gap and the average gap (EM) have similar behaviours with
26
27   respect to annealing temperature, thereby implying that the growth of the
28
29
30   native nano-crystallites and the nucleation of smaller crystallites in the
31
32   amorphous network have similar effects on the band edges and on the
33
34   conduction and valence bands. Therefore, the average gap can be used to
35
36
     describe the thermal induced changes in the optical properties of nc-Si:H.
37
38
39
40
41   4. Conclusion
42
43
44
45
46    The effect of isochronal annealing on the nano-structural and optical
47
48   properties of nc-Si:H, with the emphasis on its relation to the hydrogen
49
50   distribution and concentration, was investigated. Initial changes in the nano-
51
52
53   structure are observed after annealing at 400 C, as evident by termination of
54
55   (SiH2)n polyhydrides from the grain boundaries caused by the growth of the
56
57   native nano-crystallites. At higher temperatures, a further increase in the
58
59
60   native nano-crystallite size and the crystalline volume fraction is observed,
61
62
63
64                                                                                11
65
 1
 2
 3
 4   accompanied with the nucleation of smaller nano-crystallites and the
 5
 6   subsequent removal of hydrogen from the amorphous network. At
 7
 8
 9   temperatures  600 C the nucleation of the smaller nano-crystallites results
10
11   in a porous material with an increased optical band gap and average gap,
12
13   explained by the quantum size effect.
14
15
16
17
18   Acknowledgements
19
20    The authors acknowledge the financial assistance of the Department of
21
22   Science and Technology, the National Research Foundation and the Council
23
24
25   for Scientific and Industrial Research (Project no: HGERA2S) of South Africa.
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64                                                                              12
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Table 1
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                          TA         dRaman   fc     no     EM     E04

                         (C)         (nm)    (%)           (eV)   (eV)

                       As-dep.         3.9    53    2.750   3.11   1.88

                         400           4.7    57    2.758   3.05   1.87

                         600           5.2    59    2.668   3.03   1.84

                         700           8.4    64    2.651   3.10   1.87
List of Table and Figure Captions
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                     List of Table and Figure Captions



                     Table 1   Crystallite size, crystalline volume fraction and optical properties

                               after specific annealing temperatures



                     Fig. 1    FTIR absorption spectrum of the as-deposited sample and the

                               deconvolution of the stretching vibrations (insert)

                     Fig. 2    (a) Hydrogen concentration and (b) the structure factor as a

                               function of annealing temperature

                     Fig. 3    Raman spectra of the sample in the as-deposited state and after

                               annealing at specific temperatures

                     Fig. 4    XRD spectra of the sample in the as-deposited state and after

                               annealing at specific temperatures

                     Fig. 5    Refractive index spectra of the sample in the as-deposited state

                               and after annealing at specific temperatures

                     Fig. 6    Absorption coefficient spectra of the sample in the as-deposited

                               state and after annealing at specific temperatures

								
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